Underwater pelletizers are known which employ rotating disc cutters to cut or sever stranded polymer as it exits from the die plate of an extruder. The cutting is accomplished in a chamber full of circulating water which functions to cool the strand and also to carry away the cut pellets. The prior art disc cutters are of two types: (1) knives which extend radially from a central hub or (2) multiple blades which are attached to the ends of a spoked hub. In either case, the knives or blades are fastened to the hub by bolts. This requires access for fastening tools, such as wrenches. It also requires the hub to be relatively thick in cross section to provide adequate strength for supporting the blades. Due to the method of fastening, the space between blades is greater than is required for cutting the strand and discharging the cut pellets. Hence, the number of cutting edges is limited by the hub geometry.
Prior art disc cutters are a very open design. This allows the circulating water to remove heat from the die face quickly and sometimes causes what is known as die "freeze-off", that is, premature or excessive cooling of the extruded thermoplastic which tends to block off the extrusion orifices. To keep the die holes open, more heat is required.
As indicated above, prior art blades or knives have been attached individually to the hub. As a result, alignment with the die face is required, necessitating a run-in period before production can begin.
Moreover, the prior art blade-type cutters and knife-type cutters, because of their manner of attachment to the spoke or to the hub, tend to bend or twist under load, causing the cutting angle to vary. This tendency results in pellets which lack uniformity.
A representative prior art underwater pelletizer of the foregoing type is that described in U.S. Pat. No. 4,251,198 issued Feb. 17, 1981 to H. S. Altenberg and assigned to Gala Industries, Inc.